ok i saw you just posted. here is what you do. listen carefully.
use some nice secure bolts\nuts THAT YOU WILL BE ABLE TO REACH THROUGH THE SPEAKER HOLES TO REMOVE THIS POD! to secure that panel to the door. id prolly put some kind of squishy foam behind it to help with vibrations.
then tape off your door (about 3~5 inches past the cut you made) and get fiberglass stuff ready. get some 3M SUPER 77 spray adhesive and REMOVE the panel.
spray the BACK of the panel with the spray and let it tack up for ~2 minutes. get some REAL STRETCHY material - fleece can work, but i like thinner and SUPER stretchy crap - and apply it to the back of the board, make sure you have about 6~10 inches overhang on all sides. now secure the board to the door with your bolts. k, spray your tape that is on your door with the spray adhesive and stretch the overhanging cloth to the door panel and stcik it to the spray adheisve (make sure you wait the 2 minutes for it to tack up or it will NOT stick well!!) then proceed to soak in resin and apply maybe 2 layers of fiberglass. try and get all air bubbles OUT!!!
then you have a nice tub, trim it to a cool shape and sand the edges nice and smooth MAKE SURE TO LEAVE AT LEAST A 1\2 INCH LIP OVER THE DOOR PANEL OR IT WILL HAVE GAPS AND LOOK LIKE ****. now you can cut a nice flat piece of wood a little smaller than the hole in your door and not quite as long for the large open area, you can make it flush, or angled or whatever, and on the side where the speakers go, mount your two rings toward the listener. i like to go to the chineese food place and ask for some chop sticks (they might ask for a few dollars, i usually get 20~30 pairs)
CA glue is great for getting your mold to suspend where you want it - and i HIGHLY SUGGEST ROUND OVER BITS ON ALL EDGES OF WOOD!!! it makes it look tons better.
ok, now you have a nice tub on your door, some big ass bolts holding it on, some wood that is big and flat (or whatever you feel looks cool on the open side) and two rings on the door jamb side. get some SUPER STRETCHY material (i like consistency of panty hose, i mean super stretchy!!) and stretch from left to right and use hot glue on the BACK side of the tub lip that you made sure you had, and then go top to bottom and after you have those four spots tacked down, work your way in opposite angles. make sure you stretch it super tight and if needed, use staples to hold the fabric down to the boards\rings if it lifts up.
then apply resin and fiberglass making it as smooth as possible and then you are almost there. i assume you know how to sand and bondo and install speakers. so good luck.
this is prolly the best way to do what you are doing. and it will be a huge ass sealed pod.
i hope that helped. please dont ruin that truck.
if you need pictures, i can make some for you.